In a conventional multi-coin validator, coins pass along a path past a number of sensor coils which are each energised to produce an inductive coupling with the coin. The degree of interaction between the coin and the coil is a function of the relative size of the coin and coil, the material from which the coin is made and also its surface characteristics. Thus, by monitoring the change in impedence presented by each coil, data indicative of the coin under test can be provided. The data can be compared with information stored in a memory to determine coin denomination and authenticity.
UK Patent Specification 2 169 429 discloses coin discrimination apparatus utilising a plurality of inductive sensor coils which are each included in a respective resonant circuit. The resonant circuits are driven by a variable frequency oscillator through a multiplexer. As the coin passes a particular coil, the natural resonant frequency of the resonant circuit is altered due to the inductive coupling between the coin and the coil. The circuit is maintained at its natural resonant frequency by means of a phase locked loop which alters the frequency of the oscillator so as to track the natural resonant frequency of the resonant circuit during passage of the coin past the coil. As a result, the amplitude of the oscillatory signal developed across the resonant circuit varies substantially on a transitory basis. The amplitude deviation produced by the passage of the coin past the coil is a function of the coin denomination. It has been found that by using three coils of different sizes and configurations, three signals can be provided which uniquely characterise coins of a particular coin set e.g. the UK coin set.
The amplitude deviations produced by the three coils are digitised and compared with reference values stored in a programmable memory in order to discriminate between coins of different denominations. The "Sentinel" coin validator manufactured by Coin Controls Limited, the assignee hereof, operates in this manner.
It has been found that the amplitude deviations produced by a particular coin passing the various sensor coils, is a function of temperature and in our Sentinel validator, a thermistor is provided in each resonant circuit in order to compensate for temperature variations. Thus, the action of the thermistor is to render the amplitude deviation substantially invariant in respect to temperature.
The use of a thermistor, however, is only effective over a relatively narrow temperature range and furthermore increases the component count for the validator.
Also, it has been found that the presence or absence of metallic objects in the vicinity of the sensor coils can alter the calibration of the validator, as a result of inductive coupling between the metallic objects and the sensor coils.